Physical chemistry

Trapped gas

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Acetylene is a gas with many industrial applications. A highly efficient method to separate it from its close relation, carbon dioxide, is a promising route for purifying and storing ‘strategic’ gases in general.

Technologies for the separation and storage of gases are essential for producing clean fuels based on methane and hydrogen, for example, or for reducing air and water pollution from poisonous gases such as the oxides of carbon, nitrogen and sulphur1. Kitagawa and colleagues (Matsuda et al.), writing on page 238 of this issue2, provide a full characterization of a porous hybrid solid that retains the gas acetylene — itself a key material in the chemical and electronics industries — at room temperature and pressure, but adsorbs carbon dioxide only at high pressures and low temperatures. The possible extension of their methods to other strategically important gases makes their paper an outstanding contribution to this field of research.

Work on gas adsorption — the capacity of a solid (the adsorbent) to bind foreign gas molecules (the adsorbate) to its surface — has already provided useful results for methane and hydrogen3,4. Among the materials currently used as generic adsorbents are activated carbons, zeolites and nanotubes. The fabrication of high-capacity adsorbents tailored to particular absorbates, however, requires a better knowledge of adsorbate–adsorbent interactions at the atomic level.

In this context, the search for porous hybrid inorganic–organic solids is expanding rapidly5,6. Such solids, in which organic and inorganic molecular groups react to form a porous skeleton, have been identified as the best candidates for efficient interaction with gases because their pore dimensions and surface structure6 can be tuned by changing the organic linker molecule used. This implies potential applications not just in gas separation and storage, but also in catalysis. Adsorption occurs on the internal surface of the pores, usually through the onset of van der Waals forces between adsorbate and adsorbent (a process known as physisorption).

The amount of a gaseous species that can be adsorbed by a porous solid increases as the pressure of the gas rises, and decreases as its temperature rises. The rate of adsorption is dependent on the chemical affinity of the gas molecule for the solid wall. The technique of gas separation depends on the differing affinities of the various gas molecules for the wall: for a mixture of molecules, the larger the difference in their time of retention on the surface, the better the separation that can be achieved. The separation is very selective if, in a two-gas mixture, one gas species has an infinite time of residence on the surface — it becomes trapped — and the other gas species escapes the structure as the pressure of the remaining gas mixture decreases and the temperature rises.

Such a situation occurs in Kitagawa and colleagues' study2 to separate a mixture of acetylene and carbon dioxide. The adsorbent solid they describe (Fig. 1) has the formula Cu2(pzdc)2(pyz), where pzdc and pyz are two organic groups, pyrazine-2,3-dicarboxylate and pyrazine, respectively. The solid's three-dimensional structure captures acetylene (C2H2) at 300 kelvin — just above room temperature — whereas carbon dioxide, which has the same external dimensions as acetylene and very similar bulk properties, and which is a common impurity in industrial processes, exhibits a classical, reversible adsorption–desorption behaviour. At a pressure as low as 1.1 kilopascals (about a hundredth of atmospheric pressure), around 26 times more C2H2 is adsorbed than CO2. From an accurate structure determination at various temperatures between 270 and 340 kelvin, Kitagawa et al. localize the C2H2 molecules in the solid, and establish their interaction with sites in the wall, from both structural and first-principles calculations.

Figure 1: Acetylene caught.

A view of the porous hybrid organic–inorganic solid Cu2(pzdc)2(pyz) used by Kitagawa et al.2 to trap acetylene (C2H2). The trapped acetylene molecules (black, carbon; pink, hydrogen) are shown within the hybrid skeleton (yellow, carbon; pale blue, nitrogen; red, oxygen; grey, hydrogen; bright blue polyhedra, copper).

The first striking feature of Kitagawa and colleagues' work2 is its quality — it combines creativity, accurate characterization and energy calculations to demonstrate the incarceration behaviour of acetylene, and it also explains that behaviour. The authors use a technique known as MEM analysis to establish the electron density through the C2H2 molecule. They show that, unlike processes involving other adsorbed species that imply the presence of weak van der Waals forces, the C2H2–wall interaction proceeds through the interplay of acidic protons in the adsorbate and alkaline oxygen in the absorbent (Fig. 2). This highlights a point generally neglected until now: that the nature of the bonds in the adsorbate can also influence adsorption.

Figure 2: Acid–base interaction.

A projection of the electron-density distribution on the two-dimensional section along the molecular axis of C2H2. The electron density decreases from white to blue (see scale) through red and yellow. The continuum (yellow) of electron density between the C2H2 molecule and the oxygens of the framework indicates a noticeable acid–base interaction.

The second striking feature of the findings is the stoichiometric 1:1 ratio between the number of adsorbed C2H2 molecules and the number of available pores, which is reached very quickly at the lowest pressures. In contrast, a slow, smooth increase in the amount of CO2 adsorbed occurs as the pressure is increased. The case of acetylene is reminiscent of the situation when water is the adsorbate, where there is a whole-number molecular ratio between guest and host. Could this be a required condition for incarceration?

More generally, Kitagawa and colleagues' work2 poses new questions and opens novel perspectives. For example, what is the physical meaning of adsorption at the atomic level? The macroscopic 'adsorption isotherms' (graphs of adsorption capacity as a function of pressure at a constant temperature), and the related surface areas that they measure, are based on models of monolayer coverage of the surface by molecules — a result that, combined with others7, implies that adsorption corresponds to the fixation of molecules only on some sites of the wall. Further studies like those of Kitagawa et al., as well as computer simulations8, are required to identify the adsorption sites in porous hybrid organic–inorganic solids. This will probably lead to a more sophisticated definition of surfaces that takes into account the discontinuous nature of adsorption at the atomic level.

Kitagawa and colleagues stress that in their study the size of the adsorbate and that of the absorbent's pores are similar. Does this mean that there is an optimal size of pore for fixing a given molecule? Or are larger pores6,9 generally beneficial? At a time when attempts are being made to predict the structure of solid hybrids10, the next step, suggested by this study, will be to predict the structure of the active adsorption sites themselves. Clearly, finding the next set of answers to questions of gas storage and separation is a long-term challenge. But the potential of these answers to help in developing sustainable technologies for the benefit of humankind and the environment make this a challenge worth tackling.


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